Btrfs

I tried to have a root filesystem into btrfs for a Gandi server, and succeed after a number of trial-error. Here are my steps if it can be useful to others.

WARNING: this is EXPERIMENTAL and could cause DATA LOSS.

Create the server

On https://v4.gandi.net, create a new server [1] with e.g. 1 proc, 256 Mio RAM, 1 system disk, Ubuntu 18.04 LTS or Debian 9.

Update the packages:

apt update && apt upgrade -y

On https://v4.gandi.net, stop the server, open the page about the system disk, clone the system disk with another name, and attach this cloned disk to the server, start the server.

Convert to btrfs

Mostly from [2]

The package btrfs-progs currently does not contain btrfs-convert (this Debian bug) because the risk of data loss is too great. Then we need to compile btrfs-progs.

Update: btrfs-convert is included in Debian 10 buster backports and in Debian 11 bullseye.

Install dependencies (Ubuntu 18.04 LTS):

apt install gcc g++ libblkid-dev liblzo2-dev zlib1g-dev libzstd-dev asciidoc xmlto libattr1-dev pkg-config python3-dev python3-setuptools libext2fs-dev

OR Install dependencies (Debian 9):

apt install gcc g++ libblkid-dev liblzo2-dev zlib1g-dev libzstd-dev asciidoc xmlto libattr1-dev pkg-config python3-dev python3-setuptools e2fslibs-dev

Download and compile btrfs-progs (changelog, releases):

wget https://mirrors.edge.kernel.org/pub/linux/kernel/people/kdave/btrfs-progs/btrfs-progs-v4.20.1.tar.gz
tar xfz btrfs-progs-v4.20.1.tar.gz
cd btrfs-progs-v4.20.1
./configure --with-convert=ext2
make
make install
cd ..
rm -rf btrfs-progs-v4.20.1 btrfs-progs-v4.20.1.tar.gz

Display the active root filesystem, and you will be able to deduce what is the cloned filesystem:

ls /dev/xvd*
mount|grep xvd
blkid -o value -s TYPE /dev/xvda1 # and the same for others

Let’s say /dev/xvdb1 is the clone filesystem, we convert it to btrfs: (man 8 btrfs-convert)

fsck.ext4 -f /dev/xvdb1
btrfs-convert -p /dev/xvdb1

Here, I obtained the following error:

root@test:~# btrfs-convert -p /dev/xvda1
create btrfs filesystem:
	blocksize: 4096
	nodesize:  16384
	features:  extref, skinny-metadata (default)
creating ext2 image file
creating btrfs metadata
Unable to find block group for 0 27081]
Unable to find block group for 0
Unable to find block group for 0
ctree.c:2245: split_leaf: BUG_ON `1` triggered, value 1
btrfs-convert(+0x11b5a)[0x559c159c1b5a]
btrfs-convert(+0x1589b)[0x559c159c589b]
btrfs-convert(btrfs_search_slot+0x269)[0x559c159c6401]
btrfs-convert(btrfs_insert_empty_items+0x92)[0x559c159c7b3c]
btrfs-convert(btrfs_record_file_extent+0x1bc)[0x559c159d46b4]
btrfs-convert(record_file_blocks+0x14a)[0x559c159bfe92]
btrfs-convert(+0x10349)[0x559c159c0349]
btrfs-convert(+0x1135f)[0x559c159c135f]
btrfs-convert(main+0x1f59)[0x559c159bdefb]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe7)[0x7f4c11c3bb97]
btrfs-convert(_start+0x2a)[0x559c159bb5ca]
Aborted

According to [3] and [4], it can be worked around: (this command could take some hours, depending on disk size and number of files)

btrfs-convert -d -p /dev/xvdb1

or even:

btrfs-convert -n -d -p /dev/xvdb1

Or, if still unsuccessful, try to add free space, or if still unsuccessfull, re-compile btrfs-progs in version 4.17.1 (this last try worked for me for 50 Gio disk with 24 Gio free space, expanded after its original size was 30 Gio (=4 Gio free space)).

Then, mount the filesystem and delete the old ext4 snapshot:

mount /dev/xvdb1 /mnt
btrfs subvolume delete /mnt/ext2_saved
btrfs filesystem defrag -r /mnt # could take dozen of minuts
btrfs balance start /mnt # could take hours

Promote as root filesystem

Mostly from [5]

Enter the chrooted system:

for i in dev dev/pts proc sys; do mount --bind /$i /mnt/$i; done
chroot /mnt
blkid|grep xvdb1
vi /etc/fstab

Edit the root filesystem with: (the UUID is from the command blkid)

UUID=a74f5787-aee1-4981-b7e6-fbd3cb6ac919 /               btrfs    defaults 0       1

(in vi, type "dd" to remove a line, "i" to enter in edit mode, Esc to quit edit mode, ":x" to save and quit.)

vi /etc/grub.d/00_header

Comment the line:

#if [ -n "\${have_grubenv}" ]; then if [ -z "\${boot_once}" ]; then save_env recordfail; fi; fi

Update grub:

grub-install /dev/xvdb
update-grub

Quit:

exit
for i in dev dev/pts proc sys; do umount /mnt/$i; done
umount /mnt
exit

Define the cooked disk as boot disk

In Gandi V4 interface, stop the server, select the cooked disk as boot disk of this server (or detach the cooked disk and attach it as boot disk on another server).

Launch the server (it should correctly start, even if the /boot directory is on the main partition / (some old documents said it didn’t work because grub didn’t know the btrfs filesystem, but it is fixed now)).

df -hT

It should show something like:

/dev/xvda1     btrfs      50G   32G   19G  63% /